The present invention relates to a sintered sprocket which for example, is suitable as a sprocket for a cam shaft timing chain for an automobile, etc., and relates to a production method therefor.
As a sprocket as described above, for example, a sprocket which is compacted in the approximate shape of a sprocket by forging, which is finished by machining, and which is then subjected to carburizing and hardening, is known. In such a sprocket, although the precision of tooth surfaces and the surface pressure resistance are superior, there is a problem in that production costs are relatively high due to the forging processing which is carried out. In addition, an inexpensive sprocket which is punched from a plate material by pressing and is subjected to carburizing and hardening, is also known. In such a sprocket, although the surface pressure resistance is sufficient, there are problems in that the production costs are hardly decreased since a machining processing is needed for press fractures, and in addition, in that the precision of the tooth surfaces is lower due to warping caused by the press processing. Furthermore, a sprocket which is subjected to carburizing and hardening after a sintered material was subjected to a sizing processing, is also known. In such a sprocket, although the precision of the tooth surfaces and the production costs are advantageous, there is a problem in that the surface pressure resistance is inferior.
As described above, conventional sprockets have both advantages and disadvantages, respectively, and a sprocket in which all of the surface pressure resistance, the precision of the tooth surfaces, and the production costs are superior has been desired.
In the case in which a sprocket is produced from a sintered material, although there is a problem in the surface pressure resistance as described above, the surface resistance is reduced by voids which inevitably exist inside the material. Therefore, in order to improve the surface pressure resistance, increase in the density is considered, and the following methods are contemplated for that purpose. First, a green compact is presintered and is subjected to a sizing. Next, this presintered material is compacted by pressing, and it is sintered and subjected to a sizing. Then, these press-compactings and sizings are carried out two times, and the density of the sprocket can be thereby increased. However, the increase of the density is limited even in such a production method having many processes, and the surface pressure resistance of the tooth surface was insufficient. In addition, although the density is increased by carrying out forging on a sintered material and the surface pressure resistance is thereby improved, there is the same problem as in the above in that the production costs are relatively high.
It is an object of the present invention to provide a sintered sprocket which can improve the surface pressure resistance without an expensive forging processing, and to provide a production method therefor.
A sintered sprocket of the present invention consists of an Fe-based sintered alloy containing C at 0.2 to 0.5% by weight, Mo at 0.2 to 1.0% by weight, and Ni at 0.2 to 1.0% by weight, and exhibits a metallic structure in which an internal base layer comprises ferrite and bainite and in which the ferrite adjoins voids.
In the sintered sprocket as constructed above, since the ferrite adjoins the voids, the ferrite is plastically deformed and the voids which are located inside the tooth surface are collapsed by carrying out a deformation processing, such as a rolling, on the tooth surface. As a result, volumes of the voids are reduced, and the density in the vicinity of the tooth surface is increased. The surface pressure resistance can be thereby improved while maintaining a high-precision tooth surface and the production costs are lower by using powder metallurgy techniques. In addition, since the internal base layer contains bainite, the strength and the hardness of the matrix is increased.
The above component composition is indispensable in order to obtain the metallic composition as described above. In the following explanation, xe2x80x9c% xe2x80x9d refers as xe2x80x9c% by weightxe2x80x9d.
C: 0.2 to 0.5%
In the case in which the C content is below 0.2%, the strength and the hardness of the matrix are insufficient, since bainite is hardly formed. In contrast, in the case in which the C content exceeds 0.5%, the density is insufficiently increased when a deformation processing is carried out on the tooth surface, since the ferrite content is insufficient. Viewed in this light, it is desirable that the ferrite in the base layer be 40% or more by area.
Mo, Ni: 0.2 to 1.0%
Ni and Mo contribute to generation of bainite by strengthening the matrix in the base layer and by improving the hardening property. In the case in which the Ni content and the Mo content are below 0.2%, respectively, the above effects are insufficient. In contrast, in the case in which the Ni content and the Mo content exceed 1.0%, martensite is easily deposited and the strength is lowered. The present invention may include the case in which the martensite coexists with the bainite.
It is desirable that the ferrite in the base layer be 40% or more by area, the density after rolling is thereby further increased, and the surface pressure resistance can be improved. In addition, it is desirable that a carbide layer be provided at the frontmost location which is outside of the base layer, and the surface pressure resistance can be thereby improved. The carbide layer can be formed by a carburizing processing.
Next, the production method for the sintered sprocket of the present invention comprises rolling an Fe-based sintered alloy, and carburizing and hardening the Fe-based sintered alloy, wherein the Fe-based sintered alloy contains C at 0.2 to 0.5% by weight, Mo at 0.2 to 1.0% by weight, and Ni at 0.2 to 1.0% by weight and exhibits a metallic structure in which an internal base layer comprises ferrite and bainite and in which the ferrite adjoins voids.